Previously known carousel printers require bulky rotating pallets. The large steel dial used in the typical carousel printer has many parts, and due to its size, will change shape and size resulting in poor registration control and poor quality finished product. The circular pattern of the typical existing carousel printer cannot be expanded. The operator of a carousel printer is positioned at the back of the printing machine with poor access to the screen area. The design of carousel printers wastes valuable floor space and cannot be adapted to print roll to roll. A flat bed carousel style screen printing press with a vacuum distribution apparatus can be seen in U.S. Pat. No. 5,092,239.
Machines for multi-color silk-screen printing are also known and typically include a plurality of tables or supporting pallets sliding along an oval path with a plurality of fixed work stations. Each of the fixed stations can include a printing head or a drying head according to a predetermined sequence depending on the type of printing to be carried out and on the number of colors to be used in the printing process. This type of screen printing apparatus is disclosed in U.S. Pat. No. 5,090,311. While the oval path, sliding table screen printing apparatus overcomes many of the disadvantages of the previously known carousel type printing apparatus, it still suffers from the disadvantage of wasting valuable floor space, being difficult to expand and requiring re-registry of the article to be printed at each fixed station.
Multi-color screen printing and curing devices are also known having a shiftable printing head that carries a plurality of screen printing units that are each adapted to print an inked image of a color or texture different than the images printed by the other printing units. In this type of device, after the printing head has been lowered toward a web of material to be imprinted, each printing unit produces an inked image on respective, successively oriented defined areas of the web. The printing head is then raised, and a curing device such as an ultraviolet lamp is moved across all of the inked images on the web. Once each image has cured, the web advances one step to bring each of the defined areas of the web into registration with the next adjacent, downstream printing unit and the cycle is then repeated to build up a multi-color or multi-textured composite image. A linear feed screen printing apparatus for imprinting continuous webs of material is disclosed in U.S. Pat. No. 4,813,351. While this type of device provides linear feed of continuous links of material to be imprinted, it suffers from the disadvantages of supporting the material to be imprinted on a stationary table and pulling the material to successively advance it step to step from station to station during the printing process potentially leading to stretching of the material and improper registration of the material after it has been moved. In addition, this device suffers the disadvantage of having to physically move the printing head to make way for the application of a curing device to the inked images on the web prior to advancing the web.
In a pallet type screen printing machine, a pallet is indexed to a printing station where an article supported thereon is screen printed. A feed table with guides is typically provided to facilitate the accurate positioning of a workpiece onto the pallet at a feed station. As disclosed in U.S. Pat. No. Re. 29,206, this type of machine can include an endless drive for moving one or more pallets to and from a printing station in an endless path, where the face of the pallet is always maintained in a face up position. The pallet is formed to define a vacuum chamber which is operatively connected to a source for drawing a negative pressure thereon. A flexible coupling is required operatively connecting the pallet to the endless drive to permit limited movement of the pallet so that the pallet may be brought into accurate registration with the printing head at the printing station. Registration of the pallet in the printing station is effected by a locating cam and a cooperatively associated cam follower, which when activated, will effect the required alignment and registration of the pallet with the printing head. While this device discloses a linear feed path and application of vacuum to the workpiece at the printing station, it suffers from the disadvantage of using one or more pallets, which requires the accurate positioning of the workpiece with respect to the pallet and thereafter accurate positioning of the pallet with respect to each printing station.
Conveyorized vacuum tables have previously been proposed for feeding and supporting sheet material using a penetrable support bed and a plurality of vacuum chambers which move with the support bed and communicate with it. Vacuum is applied to the vacuum chambers to cause the conveyor to hold the sheet material in place on the conveyor during cutting or feeding operations. A conveyorized vacuum table of this type is disclosed in U.S. Pat. No. 4,646,911. Typically, conveyors of this type are manufactured with flexible belt materials, such as woven fabric, rubber or the like. These types of flexible materials are subject to stretching during use and cannot be easily adapted for reliably positioning an article carried thereon accurately with the desired degree of repeatability. This limitation has made such conveying systems unsuitable for use in conveying workpieces for screen printing devices.
A perforated steel belt conveyor is disclosed in U.S. Pat. No. 3,506,129 for separating solids from liquids. The perforated steel belt conveyor is used as an endless belt sieve. Liquid containing solids is delivered to the top surface of the belt and passes through perforations in the belt as the belt moves at a small upward incline to the horizontal and the solids remain on the belt surface.